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Other Camera functionalities

Image memory

Internal camera memory allows the user to defer transmission of an
image or a sequence of images, allowing the system to cope with
limited bus or processing bandwidth, or it can let a frame grabber
change its multiplexed input.
Simple cameras with no image memory
enable only a delayed read out of the sensor with the image quality
being affected by thermal noise.

Preprocessing

There can be benefits in performing some image preprocessing onboard
the camera: either by sending a corrected image or reducing the
amount of data that is transferred, thus reducing the CPU overhead on
the host PC. These tasks can be configured inside the camera by the
user. Some basic preprocessing operations like Bayer colour conversion,
threshol ding and run-length encoding are already available in some
cameras, but need to be configured for the individual application by the
user.

Many more advanced machine vision cameras are equipped with
onboard processors. These powerful camera systems are particularly
suitable for use in demanding imaging applications, as they remove the
need for a PC. Other advantages are their compact size and their flexible
programming capabilities. These cameras use embedded operating
systems such as µClinux (a Linux variant) with a GNU-compiler which
offers a flexible, extendable and cost-effective software platform.

Additionally, the use of FPGA technology enables the development of
imaging algorithms that can be executed in hardware. This allows
special algorithms to be developed for vertical markets and applications
which can run standalone on the camera. Possible applications for this
type of camera include the classification of vehicles according to make,
model or class, as well as special medical diagnostic applications.

Common preprocessing functions include:

Bayer to RGB or HSI conversion

Defect pixel and flat-field correction

Programmable look-up tables

Image mirroring

On-line jpeg-compression

Intelligent cameras available on the market already contain these and
further preprocessing possibilities.

Auto gain / exposure / iris

In some applications, particularly those that need to function outdoors,
the camera must produce good images in variable lighting conditions.
Auto gain control is used to alter the amplification applied to the image
in order to normalise the brightness, however, any noise in the image is
amplified together with the signal, this technique only works well for
small changes in light. Auto exposure control is similar, changing the
shutter speed to achieve the required brightness.

The auto shutter function is not suitable for all applications that need
shorter exposure times to avoid motion blur when the exposure is too
long and some of the latest cameras targeting outside use allow the
range of adjustment to be limited to prevent this. Auto iris control
requires special lenses that take video level information from the
camera and adjust the iris. Such lenses typically use an analogue output
from the camera or a DC direct drive signal to change the size of the iris
to main tain the average output from the camera at a predetermined
level.

This method works over a wide range of lighting levels, but it
should be remembered that the depth of focus will change with the
size of the iris.
Problems can arise on some cameras when these techniques are used
together, as they can work against each other. On occasion, the
brightness level may appear to 'hunt' and never settle at one point.
Some cameras have intelligent auto level modes where the priority and
range of variation and rate of change of the functions (gain, shutter or
iris) can be configured.

In recent years the introduction of the P-Iris lens has brought more
flexibility to industrial applications. By using a stepper motor the iris
position can be accurately controlled from the camera. Unlike DC and
video iris which is expecting a continuous level signal, the P-Iris holds it
position until updated. This allows the camera to be triggered which is
not possible for standard auto-iris lenses. In addition cameras can include more control of iris increasing the viability of triggered outside
applications.

Sequencing modes

Many advanced cameras include a sequencing mode. This allows a set
of predefined settings such as gain, size and location of ROI and
exposure to be stored in the camera, ready for fast switching. Sequences
of settings can be initiated by software, I/O signals or cycle round on
each trigger or to burst a sequence on a single trigger. Different cameras
feature different levels of sequence features so it is important to check
that the camera does what you require.